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Soft state is information maintained by refreshes and allowed to expire when those refreshes cease. It can be reconstructed from continuing communication with the participants that still need it. Hard state instead persists until explicitly changed or removed; a lost deletion or failed participant can therefore leave stale information indefinitely. This distinction concerns the maintenance rules, rather than simply whether the information is in volatile memory.

For example, a router can keep IP multicast membership information for a local network. Periodic membership queries and reports refresh that information; if no member reports within the prescribed interval, the membership expires. A host that crashes without unsubscribing is eventually forgotten. A lost report need not remove membership immediately, because repeated reports and a suitable expiry interval tolerate occasional loss. If the router loses its membership information, subsequent queries and reports rebuild it.^[ref]

This is attractive in Internet components because packet loss, host crashes and router restarts need not be repaired by a perfectly delivered sequence of explicit deletions. Refreshes repair some omissions, and expiry removes abandoned entries and releases resources. Components can recover local state without retaining an indefinitely accurate history of everyone else's actions. Soft state does have a cost: refresh traffic, timer storage and temporary stale or missing information, with a tradeoff between rapid expiry and tolerance of delayed refreshes. Its central benefit is